All About Tachometers

A modern electronic tachometer uses a signal from the engine to
generate a readout of engine RPM (revolutions per minute). The
readout may be a moving pointer on a calibrated scale or some digital
electronic display.

Historical development

In the beginning, tachometers were strictly mechanical. The first photo shows a Jones Portable (centrifugal) Tachometer
I have that probably dates from WWII. When the drive shaft rotates, a
set of weights is rotated through a gear train. As RPM increases,
the weights fly further apart by centrifugal force and move a collar on
their common shaft. The collar is linked to a readout dial.
The gear train affords three different ranges: 50-500, 500-5000 and
5000 to 50,000 RPM. The rotating weight mechanism recalls the
flying ball governors on steam engines. This is a fairly
delicate, expensive device - something that would have been limited to
calibrating the mechanical tachometers on the engines themselves or
setting mechanical governers. The oiled cloth cover of the Jones
tachometer case still smells like war-surplus from the 1940's to
me. I have checked this tachometer against modern electronic
reference tachometers and it is still accurate within 5%.

A tachometer is more generally useful at the engine control panel than
at the engine itself. A flexible spring-steel spirally wound cable was
used to mechanically transfer the rotation of the engine to the
tachometer. If you are old enough you might remember automotive
speedometer cables. Same thing. In the old mechanical
speedometer, a flexible rotating cable with it's ends ground square
started in the hub of a front wheel and ended in the speedometer
body. Tachometers tended to run off the end of an engine
camshaft. This was physically convenient place to attach a cable
and, as the camshaft of a four-stroke engine rotates at half the
crankshaft speed, less prone to wear. Even a two-cycle diesel,
which lacks a cylinder valve camshaft, has other coupled half-speed
shafts to drive the injection pump, water pumps or the blower. The
rotating cable was enclosed in a larger, flexible spirally-wound
shell. Lubricating the cable was a challenge. Grease packing at
the factory had to be periodically renewed through preventive
maintenance. There were aso practical lengths beyond which a
rotating cable could transmit a small amount of power against friction
between the cable and the shell.

The mechanical tachometer readout itself might consist of a magnet
rotated by the cable which transmitted force to a closely positioned
aluminum disc The disc was connected directly to the readout
pointer. A spring biased the pointer back to zero. Eddy currents were
induced into the disc by the rotating magnet. These eddy currents
opposed the magnet polarity with a force proportional to the rotation
rate. The faster the magnet rotated on the end of the cable, the more
force transmitted to the disc
and the further the pointer moved up the scale against the return
spring. There was very little to wear in this mechanism
compared to the flying ball designs.

Electric Tachometers

Problems with mechanical cable wear, and probably cost, led to the
development of the electric tachometer. A very short shaft
coupled the camshaft to a small electric permanent magnet
alternator. The shaft was shaped like the end of a tachometer
cable where it entered the engine. The sender screwed on the same
threads as the mechanical cable housing. Everything was therefore
backward and forward compatible. The sender generated an alternating
current whose power (and frequency) was proportional to engine
speed. The sender was connected by copper wires to a sensitive
meter. The faster the sender turned, the more power transferred
to the meter and the higher the pointer moved against it's dial.
Note that, although the frequency of the alternating current signal is
also proportional to rotation, the electric tachometer only used the
alternating current's power.

Some mechanics are still biased against electrical or electronic
instruments. Mechanical thermometers and pressure gauges as well
as tachometers are "calibrated" by their design tolerances in
manufacturing. They always tell the truth, or so we
believe. Poor connections and calibration differences between
senders and gauges can make electric gauges lie. A mechanical
gauge is physically coupled to and an integral part of it's sensing
apparatus. Mechanical pressure gauges have tiny capillary tubes
which transmit pressure at the engine directly to the gauge. The
difference between "electric" and "electronic" is the presence of
electronic components such as vacuum tubes or transistors.

Electronic Tachometers

The development of the transistor in the mid-1950's led to the
electronic tachometer. An electronic tachometer overcomes several
issues with electric tachometers. The electric tacho "meter" must
be fairly sensitive thus somewhat delecate and expensive. The
calibration of an electric tachometer is variable. The tight
mechanical lock provided by a rotating cable is not available.
Hence an electic tach can be either calibrated more accurately or drift
out of calibration just as well. The electronic tachometer
measures frequency. An alternating current comes from the engine
at some frequency proportional to RPM. An easy source is the
familiar tachometer sender, this time counting cycles instead of
measuring power. Tachometers also operated from magnetic pickups that
put out a pulse
each time a gear tooth rotated past. The problem here is that
different engines have different gears with 30, 113, 126, 132, 136 or
159 teeth. So a mag pickup tach tends to operate on a
significantly
higher frequency signal than tachometer sender. 400 Hz might correspond
to 3000 RPM on a signaflex mag sender and 182 RPM on just one of the
several possible mag pickup ratios.

Electronic tachometers based on vacuum tubes probably existed, I just
never saw any. They would have been far too expensive and
unreliable for mass-produced automobiles. These are two themes of
the tachometer story: reduce manufactured cost and increase
reliability. I do have a tube-based electronic stroboscope, probably
from the late 1950s, which measures RPM very accurately if a bit
ambigously. Stroboscopes are very bright, fast calibrated flashing
lights. Strobes are used to "freeze" repetitive actions by
illuminating them at the same point in every cycle. A rotating
fan appears to stand still. The ambiguity in the use of a
stroboscope for measuring RPM is that it is very hard to tell whether
something is being made visible every revolution or every other
revolution. It's pretty easy to lock on to some multiple or
sub-multiple of the rotation rate.

The magnetic sender or magnetic pickup both have a cost penalty.
The sender or pickup is specifically there to measure RPM, nothing
else. With the switch from generators to keep the starting
battery charged in the 1940's to alternators by the 1960's, another
source of tachometer signal became available. An alternator
produces alternating current at a frequency proportional to engine
speed. You normally don't think about this as the AC is
immediately rectified to DC within a modern alternator. The term
"alternator" came into vogue when the diodes to rectify the power were
still physically seperated from the alternator. The name stuck even
though power has come out of the "alternator" as direct current for 50
years. Alternators are almost universally belt driven. The
speed of an alternator is porportional to pully ratio and number of
alternator poles as well as engine speed. Still, the electronic
tachometer can be set to count pulses of AC any way it needs to.
So, with a tachometer signal available for the insignificant cost of
adding a small tab terminal to the alternator, tachometer senders and
mag pickups started to disappear. As I write this in 2011, if you
want an electronic tachometer to operate from a mag pickup or a mag
sender, it's a special order. Alternator driven tachometers have
taken over.

Tachometer Interface Issues

If you have replaced a tachometer in the last ten years you will have
learned that the process is not necessarily simple. Getting the
tachometer to read at all may be a chore if you have a magnetic sender
or a magnetic pickup. Usually you can pick a signal off the
alternator and bypass that whole issue. But getting the
tachometer to read the right number given dozens of possible
relatiuonships between RPM and alternator frequency is a
challenge. Virtually every electronic tachometer has some
complicated, mechanically flimsy way to set the ratio. First,
remember that there are no "marine" tachometers. The market only
mass-produces one tachometer for bulldozers, passenger busses or
powerboats. The tachometer, like so many other things, has been
"value engineered" to be inexpensive and as good as it needs to be but
no better. The switches and potentiometers to set up the
tachometer only do the job because they get adjusted once, at
installation. Some tachometer suppliers use tiny "dip switches"
to set the ratio. Dip switches have been abandoned in most
applications because they are notoriously noisy and unreliable.
If you can correctly read the manufacturer's chart that links pully
ratio and alternator poles to dip switch settings you are a more
patient person than me. But I digress. Lets attack this
systematically. First, lets get the dip switch reliability issue
out of the way. It's true, dip switches are bad, but there is a
trick the manufacturer employs to get past this.